Bullous pemphigoid (BP) is an autoimmune blistering disease characterized by autoantibodies targeting type XVII collagen (Col17) with the noncollagenous 16A (NC16A) ectodomain representing the immunodominant site. The role of additional extracellular targets of Col17 outside NC16A has not been unequivocally demonstrated. In this study, we showed that Col17 ectodomain‐reactive patient sera depleted in NC16A IgG induced dermal–epidermal separation in a cryosection model indicating the pathogenic potential of anti‐Col17 non‐NC16A extracellular IgG. Moreover, injection of IgG targeting the murine Col17 NC14–1 domains (downstream of NC15A, the murine homologue of human NC16A) into C57BL/6J mice resulted in erythematous skin lesions and erosions. Clinical findings were accompanied by IgG/C3 deposits along the basement membrane and subepidermal blistering with inflammatory infiltrates. Disease development was significantly reduced in either Fc‐gamma receptor (FcγR)‐ or complement‐5a receptor‐1 (C5aR1)‐deficient mice. Inhibition of the neonatal FcR (FcRn), an atypical FcγR regulating IgG homeostasis, with the murine Fc fragment IgG2c‐ABDEG, a derivative of efgartigimod, reduced anti‐NC14–1 IgG levels, resulting in ameliorated skin inflammation compared with isotype‐treated controls. These data demonstrate that the pathogenic effects of IgG targeting the Col17 domain outside human NC16A/murine NC15A are partly attributable to antibody‐mediated FcγR‐ and C5aR1 effector mechanisms while pharmacological inhibition of the FcRn represents a promising treatment for BP. The mouse model of BP will be instrumental in further investigating the role of Col17 non‐NC16A/NC15A extracellular epitopes and validating new therapies for this disease. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Laminin β4 was recently identified as a structural component of the dermal-epidermal junction and autoantigen of anti-p200 pemphigoid. In this study, we provided further evidence of the pathogenic effect of anti-laminin β4 IgG and identified potential binding partners of laminin β4. We showed that laminin β4 immune complexes led to activation of normal leukocytes and dose-dependent ROS release. Using cryosections of normal skin, we demonstrated that anti-laminin β4 patient serum IgG but not anti-laminin γ1 IgG, which are also detectable in patients with anti-p200 pemphigoid, cause dermal-epidermal separation in the presence of leukocytes. Proximity ligation assay and indirect immunofluorescence staining suggested that laminin β4 localizes closely to laminin α3 and γ2 in primary keratinocytes. Subsequent coimmunoprecipitation experiments using epidermal extracts confirmed the interaction of laminin β4 with the α3 and γ2 chains and indicated additional affinity to laminin γ1. The laminin β4-α3/β4-γ1 protein complexes were also detected using mass spectrometry. In conclusion, this study showed that anti-laminin β4 IgG can exert tissue damage in the skin, supporting their pathogenic role in anti-p200 pemphigoid. Our data further provide strong evidence for an interaction of laminin β4 with laminin α3, whereas its association to the laminin γ1 and γ2 chains is ambiguous.

Anti-p200 pemphigoid is an autoimmune blistering disease (AIBD) caused by autoantibodies against laminin β4 and/or γ1, and clinically resembles bullous pemphigoid (BP) as well as the inflammatory variant of epidermolysis bullosa acquisita (EBA). All three diseases show IgG and/or C3 deposition along the cutaneous basement membrane zone (BMZ). Although complement activation is central to BP and EBA pathogenesis, its role in anti-p200 pemphigoid remains unclear. To investigate this, we analyzed inflammatory infiltrates in lesional and perilesional skin from anti-p200 pemphigoid patients (n = 11), revealing a neutrophil-predominant pattern, with mixed neutrophil–eosinophil infiltrates in 81% of cases, which contrasted with the eosinophil-rich infiltrates typical of BP. Infiltrating neutrophils expressed C5aR1 and C5aR2. Complement fixation test (CFT) of patient sera demonstrated C3c deposition at the BMZ in 40% (20/50) of anti-p200 pemphigoid cases and 87% (13/15) of BP cases. Patients in both cohorts could be stratified into high, mild, and non-complement-fixating groups. Pharmacological inhibition of C1s (sutimlimab), C3 (compstatin), C5 (tesidolumab), or C5aR1 (avacopan) significantly blocked C3c or C5 deposition in vitro. These findings indicate that selective blockade of the classical, alternative, or terminal complement pathways effectively prevents BMZ complement deposition, highlighting pathway-specific complement inhibition as a potential therapeutic strategy for anti-p200 pemphigoid.

Pemphigoid diseases (PD) comprise a group of autoimmune disorders clinically characterized by (muco)-cutaneous inflammation and subepidermal blistering. PD are caused by autoantibodies targeting structural proteins of the dermal-epidermal junction. Skin-bound immune complexes trigger recruitment of myeloid cells, engagement of Fcγ receptors, and activation of specific kinases.

The adaptor molecule DNAX-activating protein of 12 kDa (DAP12) is broadly expressed in innate immune cells, but its role in autoimmunity remains unclear due to its dual regulatory functions. We investigated the contribution of the DAP12 pathway to bullous pemphigoid (BP), the most common autoimmune blistering disease, using a mouse model induced by transfer of anti-type XVII collagen (Col17) IgG. Repeated anti-Col17 IgG injections over 12 days produced comparable disease activity in DAP12-deficient and wildtype mice (n = 17/group), indicating that disease induction occurs independently of DAP12 signaling. Flow cytometry and immunofluorescence analysis of lesional skin further revealed a strong upregulation of the DAP12-associated triggering receptors expressed on myeloid cells (TREM) 1 in wildtype BP lesions, whereas TREM2⁺ cell frequencies in anti-Col17 IgG-treated wildtype and DAP12 knock-out animals were significantly lower than in healthy controls. Additional flow cytometry analysis demonstrated altered inflammatory infiltrates with notably reduced frequencies of Siglec-f⁺ eosinophils in DAP12-deficient vs. wildtype lesional skin. In addition, pharmacological inhibition of PI3Kδ, a downstream kinase of the DAP12/TREM pathway, did not affect disease progression in anti-Col17 IgG-induced BP. Collectively, these findings indicate that while DAP12 signaling modulates local immune cell composition, the DAP12/TREM1/2-axis does not influence overall disease activity in experimental BP.